The screen of a rear projection TV set (RPTV) has two or three layers. The
panel with a concentic circular panel, seen only if you look inside, is the
fresnel lens. It redirects the light rays to all be parallel, directly out
from the screen.

If you ever dismantle the screen of an RPTV, you must be sure to re-install
the fresnel lens with the ridged surface facing forwards. Also the fresnel
lens must be behind the lenticular lens (ribbed) panel or frosted (diffusion)
panel.

Just before reaching the screen, the light rays from the projection unit
down below are ever spreading out (diverging). The purpose of the fresnel
lens is to aim, or redirect, all of the light rays to be parallel, directly
out of the TV set.

An ordinary convex lens will do this job. But it must be as large as the
screen and it would be thick, heavy, and expensive.

A fresnel lens has the same curvatures as an ordinary lens, redirects (refracts)
the light the same way, but is collapsed down. For the RPTV, the fresnel
lens has thousands of ridges in a circular pattern and is a panel about one
eighth to one quarter of an inch thick overall.

An ordinary lens can have the curved surface facing either way, requiring
only minor calibration differences for focusing. A fresnel lens must be
positioned so the ridged surface is on the side of the parallel rays, which
means outwards for an RPTV.

Note that on the fresnel lens, some of the surfaces of the ridged side are
perpendicular to the flat surface and the other surfaces are not.

The light rays will miss the perpendicular surfaces when the flat side is
facing inwards, towards the projection units. This ensures that all of the
light rays come out parallel.

With the fresnel lens reversed, light rays will hit the perpendicular surfaces.
(An ordinary lens does not have these perpendicular surfaces.) When this
happens the rays will go off in many different directions (scatter). The
rays don't have to go far (just another 1/8 to 1/4 inch) before reaching
the front screen surface but you will see excessive haloing.

The lenticular lens is also a panel 1/8 to 1/4 inches thick. It takes some
light rays from each spot on the screen and redirects them to each side while
directing less light upwards and downwards. This gives a more even brightness
for viewers sitting off to the sides. The diffusion panel (a frosted panel
optionally used instead of the lenticular lens) does not have the graininess
caused by the rib spacing of a lenticular lens. It allows more light to travel
upwards and downwards necessitating more brightness from the projection unit
to give the viewers an equivalent picture.

Fresnel lenses come in different shapes for different purposes. Some are
equivalent to concave lenses as opposed to convex lenses. Click here for
more information: http://www.3dlens.com